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Question

# At 25∘C, the total pressure of an ideal solution obtained by mixing 3 mol of ′A′ and 2 mol of ′B′, is 184 torr. What is the vapour pressure (in torr) of pure ′B′ at the same temperature (Vapour pressure of pure ′A′ at 25∘C is 200 torr)?

A
180
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B
160
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C
140
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D
100
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Solution

## The correct option is B 160Number of moles of A, nA=3 molNumber of moles of B, nB=2 mol Mole fraction of A =No. of moles of ATotal no. of moles of A and B xA=nAnA+nB=33+2=35=0.6xB=1−0.6=0.4By Raoult's law, pA=xA×poA pB=xB×poB Ptotal=xA×poA+xB×poB184=200×0.6+p∘B×0.4184=120+p∘B×0.4 p∘B=160 torr

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